mmc: dw_mmc: remove parsing for each slot subnode

mmc: dw_mmc: remove parsing for each slot subnode

The intention to remove it comes from the conflict of
what the mmc-core does with the way dw_mmc treats disable-wp.
We could see that 'disable-wp' is supported by core but
it's deprecated by dw_mmc as we don't expect it to be existed
for each slot subnode but should be in the parent node. Based
on searching for all the upstream dts using dw_mmc, we're
confident that none of them use the deprecated way. Maybe
we should take old dtb in consideration but it was a flag day
since the time we was considering to take it away. The fact is
that there are none of dts using the deprecated way since v3.18
or even earlier. So personally I don't believe the old dtb
would/could bootup current kernel(may not?). Let's remove it now.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>